Synthesis of 3-carboxylated 2,5-dihydrothiophenes

Vinylphosphonates 11, readily prepared from diethyl carbomethoxymethyl phosphonate (10) react with α-mercaptocarbonyl compounds to afford substituted 3-carbomethoxy-2,5-dihydrothiophenes in excellent yields. The scope of this synthesis is indicated and the use of the products in the synthesis of substituted 2-carboxy-1,3-butadienes is illustrated.